U.S. patent number 6,005,395 [Application Number 08/969,432] was granted by the patent office on 1999-12-21 for method and apparatus for sensing piston position.
This patent grant is currently assigned to Case Corporation. Invention is credited to Alan D. Berger, Danley C. Chan.
United States Patent |
6,005,395 |
Chan , et al. |
December 21, 1999 |
Method and apparatus for sensing piston position
Abstract
A system for determining the position of a piston moveable
within a cylinder, or of an implement or joint, is disclosed
herein. Electromagnetic (EM) bursts such as ultra-wideband or
frequency pulses are generated and applied to a
transmitter/receiver unit. The EM bursts are launched by the
transmitter via a focusing antenna assembly from an end of the
cylinder housing towards the piston. The fluid in the cylinder
housing is in electrical communication with the piston such that a
surface of the piston represents an electrical impedance
discontinuity which causes the EM bursts to be reflected back to
the receiver. The time for the EM bursts to travel from the
transmitter to the piston and for the reflections to travel back to
the receiver via the antenna assembly is determined and converted
into a position signal representing the piston's position. A
compensation signal can be used to compensate the position signal
for variations in a parameter of the fluid within the cylinder. The
parameter may be the dielectric constant, and the variations may be
caused by factors such as temperature, contamination and fluid
type. The compensation circuit can include a pulse level analyzer,
resonance circuit, mini-dipstick, or a capacitance circuit.
Inventors: |
Chan; Danley C. (West
Burlington, IA), Berger; Alan D. (Winfield, IL) |
Assignee: |
Case Corporation (Racine,
WI)
|
Family
ID: |
25515555 |
Appl.
No.: |
08/969,432 |
Filed: |
November 12, 1997 |
Current U.S.
Class: |
324/635; 324/642;
91/167R; 92/1 |
Current CPC
Class: |
F15B
15/2869 (20130101); G01S 13/88 (20130101); G01S
13/0209 (20130101); G01D 5/48 (20130101) |
Current International
Class: |
F15B
15/00 (20060101); F15B 15/28 (20060101); G01D
5/48 (20060101); G01S 13/00 (20060101); G01S
13/02 (20060101); G01S 13/88 (20060101); G01R
027/04 () |
Field of
Search: |
;324/637,635,642,639
;92/5R ;91/167 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
|
|
266606A2 |
|
May 1988 |
|
EP |
|
31 16 333A |
|
Nov 1982 |
|
DE |
|
94 17204 U |
|
Feb 1995 |
|
DE |
|
2172995A |
|
Oct 1986 |
|
GB |
|
Other References
Brochure: Penny + Giles, Technology Leaders in Displacement
Monitoring & Manual Control Jul. 1989. .
Brochure: DC Hydrastar, Position Transducer. .
Sensors: An LVDT Primer, Jun. 1996. .
Brochure: Understanding Magnetostictive LDTS, Hydraulics &
Pneumatics, by W.D. Peterson, Feb. 1993. .
Brochure: Penny + Giles Product Data, Cylinder Transducer Model
HLP100 Oct. 1990. .
Magazine: Business Week, Not Just A Blip On The Screen, Feb. 19,
1996..
|
Primary Examiner: Regan; Maura
Attorney, Agent or Firm: Foley & Lardner
Claims
What is claimed is:
1. A method of determining the position of a piston moveable within
a cylinder housing, the cylinder housing enclosing a cavity filled
with a fluid, comprising the steps of:
generating real-time electromagnetic (EM) bursts;
launching the real-time EM bursts towards the piston through the
fluid within the cavity in the cylinder housing;
detecting real-time reflections of the EM bursts from the
piston;
generating an equivalent-time timing signal representative of the
time between launching the EM bursts and detecting the reflections,
the timing signal being of a slower time scale than that of the
real-time EM bursts and reflections; and
converting at least the equivalent-time timing signal into a
position signal representative of the position of the piston within
the cylinder housing.
2. The method of claim 1, wherein the EM bursts include
ultra-wideband pulses.
3. The method of claim 1, wherein the EM bursts include pulses
having a carrier frequency component.
4. The method of claim 1, wherein the cylinder housing includes a
first end, a second end and a side wall between the first and the
second ends, and the steps of launching the EM bursts and detecting
the reflections are performed by a transmitter and a receiver
coupled to the first end of the cylinder housing.
5. The method of claim 1, further comprising the steps of
integrating and filtering the reflections of the EM bursts.
6. The method of claim 1, further comprising the step of generating
a compensation signal responsive to a parameter of the fluid within
the cavity in the cylinder housing, and the converting step
includes converting at least the timing and compensation signals
into the position signal.
7. The method of claim 6, wherein the compensation signal is
responsive to the dielectric constant of the fluid.
8. The method of claim 7, wherein the step of generating a
compensation signal includes applying an excitation signal at
varying frequencies to a body having a cavity filled with the fluid
and determining the resonance frequency.
9. The method of claim 7, wherein the step of generating a
compensation signal includes measuring the capacitance of a pair of
conductors having a layer of the fluid therebetween.
10. The method of claim 7, wherein the step of generating a
compensation signal includes measuring the electrical impedance
discontinuity between an end of a compensation dipstick and the
fluid.
11. An apparatus for determining the position of a piston moveable
within a cylinder housing, the cylinder housing enclosing a cavity
filled with a fluid, comprising:
a generator for generating real-time EM bursts;
a transmitter coupled to the generator and to the cylinder housing,
the transmitter configured to launch the real-time EM bursts
towards the piston through the fluid within the cavity in the
cylinder housing;
a receiver coupled to the cylinder housing and configured to detect
real-time reflections after the EM bursts are reflected by the
piston towards the receiver;
a timing circuit configured to generate an equivalent-time timing
signal representative of the time for the EM bursts to travel from
the transmitter to the piston and for the reflections to travel
from the piston to the receiver, the timing signal being of a
slower time scale than that of the EM bursts and reflections;
and
a conversion circuit configured to convert at least the
equivalent-time timing signal into a position signal representative
of the position of the piston within the cylinder housing.
12. The apparatus of claim 11, wherein the EM bursts include
ultra-wideband pulses.
13. The apparatus of claim 11, wherein the EM bursts include pulses
having a carrier frequency component.
14. The apparatus of claim 11, wherein the cylinder housing
includes a first end, a second end and a side wall between the
first and the second ends, and the transmitter and the receiver are
coupled to the first end of the cylinder housing.
15. The apparatus of claim 11, further comprising:
an antenna assembly electrically coupled to the transmitter and the
receiver, the antenna assembly configured to focus the EM bursts,
to launch the EM bursts with the transmitter, and to detect the
reflected EM bursts with the receiver.
16. The apparatus of Claim 11, further comprising:
a directional sampler circuit having a first real-time port for
receiving the EM bursts from the generator, a second real-time port
coupled to the first port for sending the EM bursts to the
transmitter and for receiving the reflections, a third port coupled
to the first port and isolated from the second port for providing
an equivalent-time representation of the EM bursts, and a fourth
port coupled to the second port and isolated from the first port
for providing an equivalent-time representation of the reflections,
wherein the equivalent-time representations of the EM bursts from
the third port and the reflections from the fourth port are applied
to the timing circuit.
17. The apparatus of claim 16, wherein the timing circuit includes
a first comparator circuit coupled to the third port, a second
comparator circuit coupled to the fourth port, and a set-reset
flip-flop circuit set by the equivalent-time representation of the
EM bursts and reset by the equivalent-time representation of the
reflections.
18. The apparatus of claim 11, further comprising a compensation
circuit configured to generate a compensation signal responsive to
a parameter of the fluid within the cavity in the cylinder housing,
wherein the conversion circuit is configured to convert at least
the timing and compensation signals into the position signal.
19. The apparatus of claim 18, wherein the compensation signal is
responsive to the dielectric constant of the fluid.
20. The apparatus of claim 19, wherein the compensation circuit is
configured to apply an excitation signal at varying frequencies to
a body having a cavity filled with the fluid and to determine the
resonance frequency.
21. The apparatus of claim 19, wherein the compensation circuit
includes a pair of conductors having a layer of the fluid
therebetween.
22. The apparatus of claim 19, wherein the compensation circuit
includes:
a compensation dipstick;
a second generator coupled to the compensation dipstick and
configured to generate second EM bursts;
a second transmitter coupled to the second generator and to the
cylinder housing, the second transmitter configured to launch the
second EM bursts along the compensation dipstick towards the
piston; and
a second receiver coupled to the cylinder housing and configured to
detect second reflections after the second EM bursts are reflected
by the piston towards the second receiver.
23. An electrohydraulic control system for controlling the position
of an implement, the implement having at least one joint which is
moved by extending or retracting at least one cylinder coupled to
the joint, comprising:
an input device configured to generate command signals
representative of a commanded position of the implement;
a source of pressurized hydraulic fluid;
a valve assembly coupled between the cylinder and the source, the
valve assembly configured to control the flow of hydraulic fluid
between the cylinder and the source in response to control
signals;
a micropower impulse radar (MIR) sensor system coupled to the
cylinder and configured to sense movement of the cylinder and to
generate a position signal representative of the position of the
implement, the MIR sensor system being configured to sense the
movement using time domain reflectometry based on real-time signals
traveling through the hydraulic fluid within a cavity of the
cylinder; and
a control circuit coupled to the input device, the valve assembly,
and the MIR sensor system, the control circuit configured to
generate the control signals based upon the command signals and the
position signal and to apply the control signals to the valve
assembly.
24. The system of claim 23, wherein the implement includes an
agricultural header mounted to an agricultural harvesting
vehicle.
25. The system of claim 23, wherein the MIR sensor system
includes:
a generator for generating EM bursts;
a transmitter coupled to the generator, the transmitter configured
to launch EM bursts through a cavity in the cylinder filled with a
fluid;
a receiver configured to detect reflections after the EM bursts are
reflected by a piston in the cylinder towards the receiver;
a timing circuit configured to generate a time signal
representative of the time for the EM bursts to travel from the
transmitter to the piston and for the reflections to travel from
the piston to the receiver; and
a conversion circuit configured to convert at least the timing
signal into a position signal representative of the position of the
piston within the cylinder.
26. The system of claim 25, further comprising:
an antenna assembly electrically coupled to the transmitter and the
receiver, the antenna assembly configured to focus the EM bursts,
to launch the EM bursts with the transmitter, and to detect the
reflected EM bursts with the receiver.
Description
FIELD OF THE INVENTION
The invention generally relates to determining the position or
orientation of an implement or joint of an agricultural vehicle
(e.g., tractor, combine, etc.), or construction vehicle (e.g.,
backhoe, crane, dozer, trencher, wheeled, tracked, or skid-steer
loader, etc.). In particular, the invention relates to transmitting
electromagnetic (EM) bursts through a fluid in a hydraulic or
pneumatic cylinder used to move an implement or joint, detecting
reflections of the bursts from a piston moveable within a housing,
and determining the position of the piston based upon the time
between transmitting the bursts and detecting the reflections.
BACKGROUND OF THE INVENTION
Pneumatic and hydraulic cylinders are extensively used in actuator
assemblies for moving implements, arms, booms and other components
of mobile hydraulic machines such as tractors, combines,
excavators, dozers, loader-backhoes, etc. For example,
tractor-mounted implements such as plows are typically supported by
hitch assemblies which include hydraulic cylinders for raising and
lowering the implements. Harvesting heads on combines, blades on
dozers, and buckets on loader-backhoes are further examples of
implements typically positioned by hydraulic cylinders.
Electrohydraulic control systems for such actuator assemblies
require position feedback signals representing the positions of the
implements or mechanical joints being controlled. Some sensing
assemblies (e.g., LVDTs) which provide position feedback signals
are coupled to implements or mechanical joints using external
linkages. External sensing assemblies, however, are subject to
external impacts and other environmental influences.
The positions of implements or mechanical joints can also be
determined using sensing assemblies internal to the cylinder. The
internal sensing assemblies measure the extension of the cylinders
which move the implements or mechanical joints. Cylinder extension
is determined by measuring the position of the piston within the
cylinder housing. The piston, in turn, moves a cylinder rod coupled
to the implement or mechanical joint. The position of the implement
or the joint is then determined as a function of piston position
which depends upon the geometry of the particular mechanical
system.
Various forms of apparatus for measuring positions of pistons
within hydraulic cylinders are available. For example, the
positions of pistons within cylinders have been measured using
acoustic signals, radiofrequency (RF) signals and microwave signals
with different sensing assemblies and circuit configurations.
However, these apparatus suffer from such drawbacks as relatively
high complexity and cost, relatively low reliability, durability
and accuracy, and the need to extensively modify the cylinders to
accommodate the sensing assembly.
SUMMARY OF THE INVENTION
Accordingly, one embodiment of the present invention provides a
method of determining the position of a piston moveable within a
cylinder housing. The cylinder housing encloses a cavity filled
with a fluid. The method includes generating electromagnetic (EM)
bursts, launching the EM bursts towards the piston through the
fluid within the cavity in the cylinder housing, detecting
reflections of the EM bursts from the piston, generating a timing
signal representative of the time between launching the EM bursts
and detecting the reflections, and converting the timing signal
into a position signal representative of the position of the piston
within the cylinder housing.
Another embodiment of the present invention provides an apparatus
for determining the position of a piston moveable within a cylinder
housing wherein the cylinder housing encloses a cavity filled with
a fluid. The apparatus includes a generator for generating EM
bursts. A transmitter is coupled to the generator and to the
cylinder housing. The transmitter launches the EM bursts towards
the piston through the fluid within the cavity in the cylinder
housing with a focusing antenna assembly. A receiver is coupled to
the cylinder housing and detects reflections after the EM bursts
are reflected by the piston towards the receiver via the antenna
assembly. A timing circuit generates a timing signal representative
of the time for the EM bursts to travel from the transmitter to the
piston and for the reflections to travel from the piston to the
receiver. A conversion circuit converts the timing signal into a
position signal representative of the position of the piston within
the cylinder housing.
Another embodiment of the present invention provides an
electrohydraulic control system for controlling the position of an
implement. The implement has at least one joint which is moved by
extending or retracting at least one cylinder. The control system
includes an input device which generates command signals
representative of a commanded position of the implement. The
control system also includes a source of pressurized hydraulic
fluid. A valve assembly, which is located between the cylinder and
the source, controls the flow of hydraulic fluid between the
cylinder and the source in response to control signals. A
micropower impulse radar (MIR) sensor system coupled to the
cylinder senses movement of the cylinder and generates a position
signal representative of the position of the implement. A control
circuit is coupled to the input device, the valve assembly and the
MIR sensor system. The control circuit generates the control
signals based upon the command signals and the position signal and
applies the control signals to the valve assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will become more fully understood from the following
detailed description, taken in conjunction with the accompanying
drawings, wherein like reference numerals refer to like parts, in
which:
FIG. 1 is a block diagram illustrating an implement position
control system for an off-highway vehicle;
FIG. 2 is a block diagram showing a hydraulic cylinder and a
circuit for sensing the position of a piston moveable within the
cylinder by transmitting and receiving electromagnetic (EM) bursts
such as ultra-wideband (UWB) pulses through a fluid within a
housing surrounding the cylinder;
FIG. 3 is a block diagram showing a hydraulic cylinder and a
circuit for sensing the position of a piston moveable within the
cylinder similar to the circuit shown in FIG. 2 except for a
compensation circuit;
FIG. 4 is a block diagram showing a hydraulic cylinder and a
circuit for sensing the position of a piston moveable within the
cylinder similar to the circuit shown in FIG. 2 except for a
compensation circuit;
FIG. 5 is a block diagram showing a hydraulic cylinder and a
portion of a circuit for sensing the position of a piston moveable
within the cylinder similar to the circuit shown in FIG. 2 except
the directional sampler and the transmitter/receiver are
integral.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, an electrohydraulic control system 10 used in
an off-highway vehicle is shown. This system controls the position
or orientation of a joint or implement 12 (e.g., head, plow,
bucket, blade, etc.) of an agricultural vehicle (e.g., tractor,
combine, etc.), or construction vehicle (e.g., backhoe, crane,
dozer, trencher, wheeled, tracked or skid-steer loader, etc.).
Control system 10 controls the position of implement 12 using a
hydraulic actuator 14 supplied with pressurized hydraulic fluid
from a valve assembly 16. Valve assembly 16 receives raise and
lower signals from a control unit 18 in response to commands from
an operator interface 20. Control unit 18 can control the position
of implement 12 in a closed-loop as described below.
Control unit 18 includes a microprocessor-based circuit or a
dedicated, specific-purpose hard-wired circuit. Operator interface
20 includes operator-actuatable command devices such as
potentiometers and switches which generate command signals sent to
control unit 18 via a signal bus 22. The command signals, for
example, represent raise and lower signals, reference position
signals, raise and lower rate signals and mode select signals
(e.g., manual, replay, return-to-position, float or height control
mode). Other applications include different command devices which
generate command signals appropriate for the particular
application.
In response to the command signals from operator interface 20,
control unit 18 generates a raise signal 24 and a lower signal 26
which are applied to a raise solenoid 28 and a lower solenoid 30
mounted on valve assembly 16, respectively. Raise signal 24 and
lower signal 26 may be, for example, pulse-width-modulated (PWM)
signals. In response to signals 24 and 26, valve assembly 16
controls the flow of pressurized hydraulic fluid between a source
32 and hydraulic actuator 14. Source 32 includes a pump connected
in series with a fluid storage tank and filters (not shown). The
hydraulic fluid is transferred through conduits (e.g, hoses, tubes,
etc.) 34, 36, 38 and 40.
In the present embodiment, actuator 14 includes a hydraulic
cylinder having a cylinder housing 42 and a cylinder rod 44 which
is moved in the longitudinal direction of the cylinder by a piston
66 (shown in FIG. 2) within cylinder housing 42. By way of a
further example, a pneumatic cylinder may be used for other
applications. In pneumatic cylinder applications, a pressurized gas
is used as the fluid. The force which drives piston 66 is provided
by the pressurized hydraulic fluid supplied to actuator 14 by valve
assembly 16. The actuator 14 is connected between first and second
attachment members 46 and 48 such that changes in the piston's
position change the position or orientation of implement 12.
Actuator 14 can also be oriented in the reverse direction such that
rod 44 connects to attachment member 46 instead of 48.
Implement 12 is supported by a vehicle (not shown) using a bearing
assembly 50 including first and second bearing portions 52 and 54.
First bearing portion 52 is fixed and second bearing portion 54 is
rotatable with respect to the vehicle. Implement 12 is fastened to
second bearing portion 54 such that implement 12 is rotatable about
the axis of bearing assembly 50. First and second attachment
members 46 and 48 are connected to first bearing portion 52 and
implement 12, respectively, so changes in extension of the cylinder
cause implement 12 to rotate with respect to the vehicle.
Control unit 18 receives a position feedback signal from a position
sensing unit 56 via bus 58 and a pressure signal from a pressure
sensor 60 via bus 62. Position sensing unit 56 is coupled to
cylinder housing 42 as described below. Position sensing unit 56
replaces other position sensors such as LVDTs mounted between
implement 12 and bearing portion 52 by external linkages. Pressure
sensor 60 measures the pressure of the hydraulic fluid.
In operation, command devices within operator interface 20 send
desired command signals to control unit 18. Control unit 18
responds by generating raise and lower signals and applying them to
raise and lower solenoids 28 and 30 of valve assembly 16. Valve
assembly 16 selectively controls the flow of pressurized hydraulic
fluid from source 32 to front and rear ports of cylinder housing 42
which causes piston 66 within the cylinder to move longitudinally.
Movement of piston 66 causes cylinder rod 44 to extend or retract,
thereby changing the distance between attachment members 46 and 48.
An increased distance causes implement 12 to rotate clockwise about
bearing assembly 50 and a decreased distance causes implement 12 to
rotate in the counterclockwise direction. Extension or retraction
of the cylinder is forced by hydraulic fluid, or can be determined
by the interaction of implement 12 with the ground (e.g., in float
mode). The position of implement 12 and the pressure of the fluid
are provided to control unit 18 by position sensing unit 56 and
pressure sensor 60. Thus, the position of implement 12 can be
controlled in closed-loop fashion based upon an error signal
between the command signals from operator interface 20 and position
feedback signals from position sensing unit 56.
Control system 10, except for position sensing unit 56 and its
interface, is further described in U.S. Pat. No. 5,455,769,
commonly assigned and incorporated herein by reference. A control
system for a tractor hitch assembly is described in U.S. Pat. No.
5,421,416, and a system to move an arm on a construction vehicle
using a cylinder is described in U.S. Pat. No. 5,000,650, both
commonly assigned and incorporated herein by reference.
Position sensing system 56 determines the position or orientation
of implement 12 by measuring the position of piston 66 within
cylinder housing 42. Once the piston position is known, control
unit 18 can calculate the position or orientation of implement 12
as a function of piston position and the geometrical parameters of
the machine system. Position sensing system 56 includes a
micropower impulse radar (MIR) as explained below.
FIG. 2 shows hydraulic actuator 14 and an MIR circuit 64 for
sensing the position of piston 66 by transmitting and receiving
electromagnetic (EM) pulses or bursts through the pressurized
hydraulic fluid within a cavity 70 in cylinder housing 42. A
sequence of EM bursts are generated by a generator 72 and applied
to a directional sampler 74 via a transmission line 76.
Preferably, the EM bursts are ultra-wideband (UWB) or square-wave
pulses. U.S. Pat. No. 5,457,394, herein incorporated by reference,
describes a circuit including a generator of UWB pulses which are
200 psec rise time square-wave pulses repeated at a
pulse-repetition interval (PRI) of 1 Mhz. However, other pulse
widths and PRIs can be used. The UWB pulses are repeated to allow
the integration or averaging of approximately 10,000 reflected
pulses for increased noise immunity. Noise immunity can be further
increased by modulating (e.g., dithering or randomizing) the
pulses. The UWB pulses are unlike acoustic, RF and microwave
signals since they are a sequence of impulses having no carrier
frequency. No specific frequency is associated with the UWB pulses,
and the frequency spectrum is related by the Fourier transform of
the pulses. The UWB term refers to the wide spectrum of frequencies
comprising the pulses. A timing generator with a crystal oscillator
for high accuracy is described in U.S. Pat. No. 5,563,605,
incorporated herein by reference. A circuit used to generate a GATE
input signal for sampler 74 is also described in this patent.
The EM bursts, however, are not limited to square pulses and bursts
may include pulses having any shape and form, including pulses
generated by a highly-focused antenna. For example, the pulses may
include sine-wave signals or a combination of sine-wave signals
having a carrier frequency component in the RF, microwave, etc.
signal range. The frequency pulses are repeated at a predetermined
frequency less than the frequency of the carrier. Although the
sampling circuit described below can process frequency pulses, the
circuitry for generating frequency pulses is more complex and
expensive than the circuitry used to generate square pulses. The
remainder of this description assumes that the EM bursts are UWB
pulses. However, as noted, pulses having a frequency component can
also be used.
Directional sampler 74 includes four ports. Ports 1 and 2 are
"real-time" bidirectional ports. Port 1 receives the UWB transmit
(T) pulses from pulse generator 72 and couples the T pulses to port
2. Port 2 transmits the real-time T pulses through interconnect
cable 77 to a transmitter/receiver unit 78, and receives a portion
of the T pulses as real-time reflected (R) pulses from unit 78.
Ports 3 and 4 are sampled "equivalent-time" ports that output
signals from a differential sampler within directional sampler 74.
Thus, ports 3 and 4 are not bidirectional. Port 3 is coupled to
port 1 in equivalent time and is isolated from the R pulses at port
2. Port 3 generates an equivalent-time replica of the T pulses at
port 1. For example, when the real-time T pulses at port 1 have a
200 psec pulse-width, the equivalent-time replica of the T pulses
appearing at port 3 have a pulse width of 200 usec. Port 4 is
coupled to port 2 in equivalent time and is isolated from the T
pulses at port 1. Port 4 generates an equivalent-time replica of
the R pulses at port 2. Thus, equivalent-time replicas of the T and
R signals at ports 1 and 2 appear at ports 3 and 4.
The isolation between port 3 and the R pulses and between port 4
and the T pulses allows directional sampler 74 to accurately
distinguish the T pulses from the R pulses even when the T and R
pulses overlap, such as when piston 66 is close to
transmitter/receiver unit 78. Directional sampler 74 is further
described in U.S. Pat. No. 5,517,198, incorporated herein by
reference.
Transmitter/receiver unit 78 is coupled to the rear end 80 of
cylinder housing 42. Cylinder housing 42 also has a front end 82
and a cylindrical side wall 84 between ends 80 and 82. As shown in
FIG. 2, transmitter/receiver unit 78 can be coupled to rear end 80
substantially at the longitudinal axis of the cylinder.
Transmitter/receiver unit 78 is in electrical communication with
the fluid via an antenna assembly 86 supported by rear end 80.
Antenna assembly 86 consists of separate transmitting and receiving
antennas, or it may consist of a single antenna used for both
transmitting and receiving. The hydraulic fluid makes electrical
contact with antenna assembly 86 such that the T pulses applied to
the transmitter of unit 78 are focused and launched into the
pressurized fluid towards piston 66. In one embodiment of the
present invention, the cylinder is of a relatively short and wide
geometry so as to limit the reflections of the T pulses off side
walls 84 (e.g., the cylinder is approximately >=6" diameter and
<=6" length).
Transmitter/receiver unit 78 can be integral (i.e., one-piece) with
antenna assembly 86, or can be separate and connected by wires. An
integral unit may be more cost-effective, and a separate unit may
increase flexibility in mounting unit 78. The transmitter is
described in U.S. Pat. Nos. 5,457,394 and 5,517,198, incorporated
herein by reference, and the receiver is described in U.S. Pat.
Nos. 5,523,760 and 5,345,471, incorporated herein by reference.
Piston 66 includes a rear surface 88 and a front surface 90. As
pulses from antenna assembly 86 reach rear surface 88, the
electrical impedance discontinuity between piston 66 and the
adjacent hydraulic fluid causes reflections which travel back
through the fluid to unit 78. The time for a transmitted pulse to
travel from the transmitter to piston 66 and for a reflected pulse
to travel back to the receiver depends upon the position of piston
66. Cylinder rod 44 includes a free end 92 which may be attached to
a rod eye 94 for attachment to implement 12. However, rod eye 94
may be replaced with any suitable mechanical interface for
transferring force to the implement, arm or boom being moved.
The equivalent-time T and R signals appearing at ports 3 and 4 of
directional sampler 74 are applied via lines 96 and 98 to a
directional set/reset circuit 100. Circuit 100 includes first and
second threshold comparators 102 and 104, and a set/reset flip-flop
106. Threshold comparators 102 and 104 compare the equivalent-time
replicas of the T and R signals with voltage references -V.sub.REF
and +V.sub.REF that have voltage levels of about half of the peak
amplitudes of the T and R signals, respectively. The outputs of
comparators 102 and 104 set and reset flip-flop 106, respectively.
Flip-flop 106 outputs a variable-width range pulse on line 108.
In operation, real-time T pulses from pulse generator 72 are
applied to port 1 of directional sampler 74, coupled in real-time
to port 2, and applied to transmitter/receiver unit 78. The T
pulses are launched towards piston 66 through the pressurized
hydraulic fluid within cavity 70 and are reflected by the impedance
discontinuity at piston 66. The reflected pulses are detected by
the receiver as real-time R pulses. Equivalent-time replicas of the
real-time T and R pulses appear at ports 3 and 4 of directional
sampler 74. The replicas are applied to directional set/reset
circuit 100 such that flip-flop 106 is set by the equivalent-time T
signal and reset by the equivalent-time R signal. The pulse width
of the output from flip-flop 106 represents the time for UWB pulses
to travel from the transmitter to piston 66 and for the reflected
pulses to travel from piston 66 back to the receiver.
Circuit 64 is accurate even when the T and R pulses are close or
even overlap in time. The reflected R signal is isolated from port
3 and cannot set flip-flop 106, and the transmitted T signal is
isolated from port 4 and cannot reset flip-flop 106. Thus, the
position of piston 66 within the cylinder is measured accurately
even when piston 66 is close to antenna assembly 86.
The variable-width range pulse on line 108 is converted to a
position signal 110 by a piston range conversion circuit 112. In
one embodiment, conversion circuit 112 includes a range counter
gated by the variable-width range pulse. If the equivalent-time
range scale is 1 msec=1 inch and the clock speed of the counter is
1 Mhz, the range counter will record 1000 counts/inch for a
resolution of 0.001 inch/count. The counter value can be read by a
microprocessor or microcontroller and converted into a scaled
digital position signal for use by a control algorithm.
In another embodiment, conversion circuit 112 includes analog
circuitry to convert the variable-width range pulse into an analog
signal (e.g., a DC voltage or current, PWM signal or another type
of electrical signal) which represents the position of piston 66.
Interface circuits to convert a variable-width pulse into various
types of electrical signals are known, and may provide a less
expensive circuit than a microprocessor circuit.
Thus, circuit 64 operates in the time domain since the position of
piston 66 is measured by the equivalent-time signal on line 108.
This is true regardless of whether the EM bursts include UWB pulses
or pulses having a frequency component. Operation in the time
domain simplifies the circuitry compared with operation in the
frequency domain (e.g., measuring piston position by determining
the resonance frequency of a cavity formed by the piston and the
cylinder).
In pneumatic cylinder applications, the electrical parameters of
the air within the cylinder are relatively stable with respect to
temperature. Thus, the piston position signal may not need to be
compensated for variations in the air's characteristics. In
hydraulic cylinder applications, however, electrical parameters of
the oil within the cylinder affect the speed at which the
transmitted and reflected pulses travel. In particular, the speed
of the pulses depends upon the dielectric constant of the
surrounding fluid, which in turn depends upon factors such as the
temperature of the fluid, contamination of the fluid, and type of
fluid. Thus, the piston position signal is compensated to account
for variations in the dielectric constant of the fluid.
Referring still to FIG. 2, the dielectric constant of the hydraulic
oil can be detected using a compensation circuit 114. In this
embodiment, compensation circuit 114 receives the real-time T
signal on line 116 from unit 78 and generates a compensation signal
118 applied to conversion circuit 112. The real-time T signal can
be tapped at antenna assembly 86, or at an internal node of unit
78. Compensation circuit 114 can include a pulse level analyzer
(PLA) to measure the amplitude or level of the T signal. The PLA
includes, for example, a peak-level detector which locks onto the
peak voltage of the T signal. Since the energy per T pulse is
constant, electrical characteristics (i.e., dielectric constant) of
the fluid surrounding the launching plate affects the voltage
caused by the launched constant-energy pulse. The output of the
peak-level detector is used to generate compensation signal 118
representing the peak voltage. Compensation signal 118 may be, for
example, a voltage or a current. Thus, compensation signal 118 is
responsive to the dielectric constant of the hydraulic fluid.
Other implementations of compensation circuit 114 can be used to
generate compensation signals representing the dielectric constant
of the fluid. For example, compensation circuit 114 can generate
excitation signals at varying frequencies, apply the excitation
signals to a body having a cavity filled with the fluid, and
determine the frequency at which the cylinder resonates. The
resonance frequency depends on the dielectric constant. The body
may have a cylindrical, rectangular or other shape having a
geometry conducive to producing and measuring electromagnetic
fields. The body may be placed within cylinder housing 42, or
elsewhere within the hydraulic system where it will be filled with
fluid-preferably with a dielectric constant equal or close to that
of the fluid within the cylinder.
Other compensation circuits 114 can also generate compensation
signals based upon other parameters of the fluid which affect the
speed of the pulses through the pressurized hydraulic fluid. For
example, compensation signals can be determined directly from the
fluid temperature using signals from a thermocouple, RTD,
thermistor or other temperature sensor.
When conversion circuit 112 is microprocessor-based, compensation
signal 118 can be converted into a digital signal using an
appropriate interface (e.g., an A/D converter). The digital
compensation signal is used to modify position signal 110.
Depending upon the system and application, algorithms or tables for
performing this conversion are determined by calibrating the system
and, possibly, using appropriate curve fitting algorithms, fuzzy
logic, or other well-known techniques.
When conversion circuit 112 does not include a microprocessor,
compensation signal 118 is used to modify the signal generated from
the variable-width pulse from flip-flop 106. For example, if the
variable-width pulse and compensation signals have been converted
into DC voltages, then conversion circuit 112 can adjust the DC
voltage range signal using the DC voltage compensation signal to
generate a compensated piston position signal.
Referring to FIG. 3, another MIR system for sensing the position of
piston 66 is shown. This embodiment is similar to the embodiment
shown in FIG. 2 except the compensation circuit comprises a
multiplexer or switch 120, a second transmitter/receiver unit 122,
a second pulse launcher 124, and a compensation dipstick 126. Unit
122 is the same or similar to unit 78. Compensation dipstick 126 is
a transmission guide of known length (e.g., 1 or 2 inches) which
does not interfere with the movement of piston 66. Port 2 of
directional sampler 74 is selectively connected to
transmitter/receiver units 78 and 122 by switch 120 under the
control of a select signal 128 from piston range conversion circuit
112.
When port 2 is connected to transmitter/receiver unit 78, the
operation is as described above. However, when port 2 is connected
to transmitter/receiver unit 122, the transmitted UWB pulses are
launched via pulse launcher 124 along compensation dipstick 126.
The UWB pulses are reflected by the electrical impedance
discontinuity at the interface between an end 130 of compensation
dipstick 126 and the fluid. The reflections detected by the
receiver in unit 122 are processed by directional sampler 74 and
directional set/reset circuit 100 in the manner described above.
Since the length of compensation dipstick 126 is known, the pulse
width on line 108 is a measure of the dielectric constant of the
fluid. For example, the pulse width could be used as an input to an
empirically-determined table or equation which correlates pulse
width to dielectric constant.
FIG. 4 shows an alternative MIR system for sensing the position of
piston 66 within the cylinder using a capacitor circuit 132 to
generate compensation signal 118 instead of using a PLA circuit
which analyzes T pulses as described in FIG. 2. Capacitor circuit
132 is coupled to compensation circuit 114 and mounted within the
cylinder near or adjacent to antenna assembly 86. Capacitor circuit
132 includes a pair or metal plates separated by a thin layer
(e.g., about 1 mm) of hydraulic fluid. Alternatively, capacitor
circuit 132 includes two concentric cylinders separated by fluid.
Compensation circuit 114 is configured to measure the capacitance
of capacitor circuit 132 which depends upon the dielectric constant
of the hydraulic fluid between the capacitor plates. The
capacitance signal is used by piston range conversion circuit 112
as a measure of the dielectric constant of the fluid.
Capacitor circuit 132 can also be mounted within a hose which
supplies fluid to actuator 14, or within a shunt which receives
only a portion of the fluid flowing through the hose. Measuring the
dielectric constant of the fluid outside of the cylinder is
advantageous because it minimizes modifications to the cylinder.
However, temperature differences between fluid within and without
the cylinder may adversely impact the accuracy of the compensation
signal and, therefore, the accuracy of the piston position
signal.
Referring to FIG. 5, another embodiment of part of an MIR system
for sensing the position of piston 66 is shown. This embodiment is
similar to the embodiment of FIG. 2 except directional sampler 74
and transmitter/receiver unit 78 are integral (i.e.,
one-piece).
While the embodiments illustrated in the FIGURES and described
above are presently preferred, it should be understood that these
embodiments are offered by way of example only. The invention is
not intended to be limited to any particular embodiment, but is
intended to extend to various modifications that nevertheless fall
within the scope of the appended claims.
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